But I realized what I was doing wrong. The little detail is Cubase lists an ‘extra’ (at least in my opinion) bus labeled “UR816C FX Bus”, which I carelessly selected without reflecting on what it might be, just thinking it was the only one labeled “Bus”, so assumed it was a main Bus… wrong. It seems to be exclusively taken from the output of the Rev-X channel (or Bus… rightfully so). So of course, it refers to such bus in whichever Mix you have Rev-X enabled for.
So simply selecting Mix 1, where Rev-X is disabled (in my case), on Cubase as my Main Bus did the trick. All is good now.